The most recent developments in the field of nuclear fuels have concerned particulate material in the form of microspheres or powder or sintered pellets of uranium or plutonium oxide, carbide or nitride, or even pellets containing actinides other than uranium or plutonium. This particulate material, whether sintered or not, is contained in tubes of a metal alloy of the type generally used for the manufacture of nuclear fuel sheaths, having a length of 1 to 3 meters and a diameter of 6 to 14 mm, filled to a certain level, the remaining space being designed to receive the gas released during fission of the fuel. The portion of the tube containing the fuel is closed by respective microporous retaining elements disposed at both ends. This element must be made of a material which is compatible with the fuel and with the metal alloy of the tube. This material is preferably refractory and non-reactive. It must be possible for this element to slide in the tube while ensuring complete closure of the portion of the tube containing the fuel and the element must be porous in order to enable the axial diffusion of gas through the microporous retaining element both during filling of the tube and to enable the gaseous fission products to escape into the chamber designed to receive them. The pore size must be less than the size of the smallest particles, approximately 20 to 30.mu., in order to prevent any fuel leakage from the portion of the tube in which it is contained during transport and handling as well as during filling during which the tube is subjected to vibrations of 20 to 100 Hz.